IR radiation detectors provide an electrical output which is a measure of incident IR radiation. One particularly useful IR detector is a photovoltaic (PV) detector fabricated from Group II-VI radiation absorbing semiconductor material, such as mercury-cadmium-telluride (HgCdTe). HgCdTe detectors are typically fabricated as linear and two-dimensional arrays, also known as Focal Plane Arrays (FPAs).
As seen in FIG. 1, generally a transparent substrate 1 supports a radiation absorbing semiconductor layer 2a having a first electrical conductivity. A second semiconductor layer 2b of opposite electrical conductivity forms a p-n junction with the first layer.
The array may be differentiated into a plurality of p-n junctions by selectively etching the semiconductor layers 2b and 2a, resulting in the formation of a plurality of upstanding "mesa" structures, each of which contains a radiation detecting element, or pixel. One such mesa structure 3 is shown in FIG. 1, the mesa structure 3 being contained within an etched depression or trench 3a having sidewalls 3b. The mesa structure 3 itself has surrounding sidewalls 3c.
The array typically also includes a layer of passivation applied to outer surfaces so as to reduce surface states and the resulting noise signals that detrimentally affect the operation of the p-n junction. An anti-reflection (AR) coating may also be applied over the passivation layer to reduce reflections of incident radiation.
The mesa structure 3 has an electrical contact that is typically provided in the form of one or more square or round metal pads 4. The contact pad(s) 4 provide electrical contact for external read-out circuitry (not shown), usually via an indium "bump" interconnect 5, to the p-n junction. The external read-out circuitry is bonded to the indium bumps 5 to form a hybridized FPA/read-out assembly.
If radiation enters the array through the bottom surface of the substrate, that is, the surface opposite to the surface that supports the radiation absorbing semiconductor layer, the array is referred to as a "backside-illuminated" array.
It is conventional practice to scan incident radiation over the array, particularly linear arrays, with a rotating mirror or the like. The direction of the scanned radiation is referred to as a scan axis, and a cross-scan axis is defined to be an axis perpendicular to the scan axis. It is also conventional practice to tilt or rotate the detector array about an axis such that a radiation receiving surface of the array is inclined at an angle to incident radiation. This rotation of the array may be accomplished for both scanned and unscanned, or "staring", detector arrays.
A problem that is presented during the use of such detector arrays results from reflection of radiation from the edges of the contact pads 4, sidewall surfaces 3b and 3c, and other top-side surface structures. This reflected radiation is radiation that first passes unabsorbed through the substrate 1, the radiation absorbing semiconductor layer 2a, and the overlying layer 2b of opposite conductivity. This unabsorbed radiation eventually encounters the array top-side edges and features and is reflected therefrom back through the body of the array. If the reflected radiation is not absorbed during the second pass through the array, the radiation emerges from the bottom surface of the substrate 1 and may propagate back into space. This propagating radiation signal is often referred to as a "light signature" (LS).
In order to minimize the light signature originating from the trench walls 3b and mesa sidewalls 3c, the wall widths must be reduced to be much smaller than an optical blur diameter. The optical blur diameter is given by 1.22 times the wavelength, divided by the numerical aperture, as described by W. J. Smith, "Modern Optical Engineering", pages 138-140 (McGraw-Hill, Inc., 1966). In general, as the feature width is decreased along the cross-scan axis, the magnitude of the light signature also decreases. Thus, the cross-scan width of a feature is proportional to its light signature. A leveling off of the light signature occurs at a feature width/optical blur diameter of unity.
One possible method to achieve the reduction in mesa wall widths would be to miniaturize the mesa diode. However, there are limitations as to a minimum useful diode size. Specially designed absorptive coatings may also be employed to reduce the light signature, although the use of such coatings complicates the array fabrication process.
What is thus an object of this invention to provide is a radiation detector having a novel metalization routing technique, in combination with a mesa structure that is differentiated into a plurality parallel-connected sub-mesas, each sub-mesa containing a PV diode.
A further object of this invention is to provide an array of radiation detectors wherein contact pad terminators and indium bump interconnects are physically located away from the illuminated active areas of the radiation detectors, such as at a periphery of the array.